Compatibility

Favicons work on all modern browsers and favicons from the faviconist editor will work everywhere.
But the favicon.js library is a different story ...

Changing the favicon works on all versions of Firefox, Opera, and Chrome.
Unfortunately, Internet Explorer and Safari don't support favicon manipulation.

Setting badge text works on all versions of Chrome, and modern versions of Firefox and Opera
(it requires canvas support and all of these browsers have supported canvas for several years now).

Setting and animating the title works on all major browsers.

History

I discovered that favicons could be
dynamically manipulated in 2006 and released the first version of favicon.js
along with the announcement. Over the years, it's evolved to include title
setting, rotation of images and scrolling of the title, and badge text for
notifications.

Since that discovery, we have seen many interesting applications of dynamic
favicons, such as:

There's even a nefarious use for it: Tabnapping.
It's not endorsed here! Well, except for April Fools jokes involving Mr.
T. But it's a fascinating example of unintended consequences. This is a
"phishing" attack in which the site quietly metamorphizes itself while
the user is in another tab.